This invention concerns a method of calibrating a partial discharge measuring device, a method of locating a flaw on a cable by analysing partial discharges, and corresponding systems.
To measure so-called partial discharges (PD) according to Standard IEC 60270, calibrating the measuring device before the actual measurement is prescribed and necessary. For calibration, a calibrating device is usually connected between the test object and earth, and feeds simulated partial discharge pulses of known charge into the test object. The partial discharge measuring device is set to a frequency band with low environmental interference levels, and is adjusted for this frequency band so that it displays precisely the charge which is fed in by the calibrating device. If the width or position of the frequency band changes, recalibration is usually required. The pulse form for partial discharge calibration is prescribed in Standard IEC 60270, and comes close to a Dirac pulse, i.e. it involves short pulses of high amplitude with a broad frequency spectrum. The pulses are repeated at short intervals, to obtain a regular display on the measuring device. The partial discharge calibration signal can therefore be considered as a periodic signal, and a corresponding crest factor, which gives the ratio of peak values to root mean square values, can be calculated. In the case of the calibration signals described above, with pulse shape and high amplitude, the result is very high values for the crest factor. This means that the partial discharge measuring device to be calibrated must have a measuring range with high dynamics and high precision.
The measuring signal is usually extracted using coupling capacitors which are attached for the measurement, or using capacitances which are inherently present in the system, e.g. capacitive implementations of transformers or circuit breakers or capacitive coatings which are present between the shield and core of power cables. The calibrating device which is connected for calibration is usually removed after calibration, since it cannot withstand the test voltage. A test voltage is then applied to the test object, and the actual partial discharge measurement is carried out.
In the case of a partial discharge measurement on a cable, the distance from a measuring device, which is usually at a cable end, to an imperfection at a fault location in the cable can usually be determined indirectly by determining signal transit times of interfering pulses. The fault location is usually determined by applying a test voltage to the cable. At the fault location, partial discharge pulses then occur. The pulses have a certain transit time until they reach the measuring device. Because, at the cable end, for each pulse there is an echo, the difference of the transit times between pulse and echo can be measured. If the fault location is very near the end of the cable, the transit time difference between pulse and echo is small. A large transit time difference indicates a fault near the measuring point. To be able to calculate precisely where the fault location is, the speed at which an interfering pulse is propagated in the cable must be known. This can be determined by means of a calibration pulse, which is preferably fed into the near cable end at the measuring device and reflected at the far end of the cable. The transit time is calculated, and the speed of the pulse propagation is determined from it. The speed depends on the temperature of the cable and the measurement frequency window under consideration.
From the above description, it becomes clear that calibration of a partial discharge measuring device according to the prior art is very resource-intensive, since the partial discharge measuring device must be recalibrated every time the frequency band is changed or the temperature of the test object changes. For calibration, each time, the test voltage must be switched off, the calibrating device must be connected, and after calibration and before the actual partial discharge measurement it must be removed again. Additionally, in the case of the calibration method according to the prior art, the requirements regarding the dynamics on measuring circuits of the partial discharge measuring device are high. The same applies to current methods of locating faults on cables.